The present invention relates to a process for the preparation of aminoarylsulphonic acids from an arylamine and a sulphonating agent in the presence of tetramethylene sulfone (sulfolane).
It is already known that aromatic aminosulphonic acids are obtained at high temperatures from an arylamine and sulphuric acid (Helv. Chim. Acta. 15, 1372 (1932)). This so-called baking process, in which the corresponding arylammonium sulphate is formed in a first stage, is afflicted by caking phenomena and local superheating, which lead to charring in some cases, undesirable side reactions and hence to reductions in the quality of the product, in particular the color of the product, and to increased expenses for the working-up and purification. Attempts have therefore been made to carry out this baking process in the presence of a solvent or a diluent, to eliminate the defects described or at least to limit them. Examples of solvents proposed for this purpose are o-dichlorobenzene (Ind. Eng. Chem. 42, 1746 (1950)) and diphenyl sulphone (Trav. Soc. Sci. Lettres Wroclaw Ser. B. 61, 5 (1953), cited in C.A. 48, 7568 i (1954)). However, the use of o-dichlorobenzene incurs long reaction times of about 15 to 20 hours and, associated with it, high energy costs. The use of diphenyl sulphone in the baking process produces high yields in some cases, but in some cases it likewise requires long reaction times and it is characterized above all by an involved separation process for the diphenyl sulphone. Because of the high melting point of diphenyl sulphone, which is over 100.degree. C., this working-up cannot be carried out by a simple filtration, but it is possible only by an extraction with benzene, and it is additionally afflicted by the need to recrystallize diphenyl sulphone before it is re-used. The solvents or diluents mentioned here by way of example have the disadvantage in common that, even in their presence, caking phenomena followed by product discolorations cannot be completely avoided, so that purification operations for the product obtained are as necessary as before.